1. Field of the Invention
The present invention relates to a solution containing a metal component, and a method of and an apparatus for forming a thin metal film, and more particularly to a solution containing a metal component for use in forming a conductive thin metal film on a semiconductor substrate of silicon or the like, and a method of and an apparatus for forming a thin metal film using such a solution, and a method of and an apparatus for forming a thin metal film in embedding a conductive metal such as copper (Cu) or the like in minute interconnection recesses defined in the surface of a substrate of silicon or the like thereby forming interconnections.
2. Description of the Related Art
Aluminum or aluminum alloys are generally used as metal materials for forming interconnection circuits on semiconductor substrates. One recent trend is to use copper as such a metal material for forming interconnection circuits. Since copper has an electric resistivity of 1.72 μΩcm which is about 40% lower than aluminum, it is more effective to prevent signal delays. In addition, because copper has much better electromigration resistance than presently available aluminum and can be embedded more easily into minute recesses by the dual damascene process than aluminum, it allows complex and minute multilayer interconnection structures to be manufactured relatively inexpensively.
Metal such as copper or the like can be embedded into interconnection grooves and via holes by the dual damascene process according to three methods, i.e., CVD, sputtering reflow, and plating. Of these methods, the plating method has a stronger tendency to be able to form highly conductive paths with a relatively easy and inexpensive process, because conductive material can be embedded more easily into minute recesses by the plating, making it customary to incorporate a design rule of 0.18 μm generation into semiconductor mass-production lines.
FIGS. 16A through 16C show a basic process for plating the surface of a semiconductor substrate with copper to produce a semiconductor device with copper interconnections. As shown in FIG. 16A, an insulating film 2 of SiO2 is deposited on a conductive layer 1a on a semiconductor base 1 with semiconductor elements formed thereon. A small recess 5 comprising a contact hole 3 and an interconnection groove 4 is formed in the insulating film 2 by lithography and etching. A diffusion barrier layer 6 made of TaN or the like and a base film 7 as a seed layer for supplying an electric current for electroplating are successively formed on the surface formed so far.
As shown in FIG. 16B, the entire surface of the substrate 18 is plated with copper according to electrolytic copper plating to fill the recess 5 with a copper layer 8 and deposit a copper layer 8 on the base film 7. Thereafter, the surface formed so far is polished by chemical mechanical polishing (CMP) to remove the base film 7, the copper layer 8 thereon, and the diffusion barrier layer 6, and planarize the copper layer 8 filled in the contact hole 3 and the interconnection groove 4 flush with the insulating film 2. In this manner, an embedded interconnection made of the copper layer 8 is formed as shown in FIG. 16C.
The base film 7 (seed layer) is formed prior to the electrolytic copper plating because its surface will serve as an electric cathode for supplying a sufficient current to reduce metal ions by reduction in the electrolytic solution and to precipitate them as a metal solid. The surface of the substrate may be plated with copper according to electroless copper plating. According to the electroless copper plating, it is the widespread practice to employ a catalytic layer as the base film 7 instead of the seed layer.
Other general known method of forming a conductive thin metal film on a ceramic substrate comprises the steps of coating (printing) a metal paste such as an Ag—Pd-based paste, a silver-based paste, or the like on the surface of the substrate, and then baking the coated metal paste. The metal paste is generally in the form of a solution that comprises a metal powder of silver, copper, or the like and a resin or glass component which are dispersed in an organic solvent. The resin or glass component enables the paste to be formed as a film, and the particles of the metal powder are held in point-to-point contact to make the thin metal film electrically conductive.
To meet demands in recent years for higher-speed and finer-circuit semiconductor devices, there is a need for growing films of materials that are poorly evaporated by CVD, and a pattern is produced which is too small to be embedded by sputtering. While the plating technology is inexpensive and technically highly complete, the electrolytic plating process allows films to be grown on only conductive materials, and the electroless plating process is open to environmental pollution as materials contained in the plating solution adversely affect the natural environment and the working and labor environment. For these reasons, there has been a strong demand for a new film growing technique in place of the conventional film growing technique.
If a thin metal film is formed of a conventional metal paste as interconnections on a ceramic substrate, then the conductivity is limited to a certain level because the thin metal film is rendered conductive by point-to-point contact between metal particles. To increase the conductivity, the thickness of the thin metal film needs to be increased to provide more sites of-point-to-point contact between metal particles. The thicker metal film is more expensive to manufacture.
There has been developed a dispersion liquid comprising ultrafine metal particles dispersed in an organic solvent. However, available processes of producing ultrafine metal particles are of low productivity. One example of such processes is a gas evaporation process in which is metal is evaporated under vacuum in the presence of a small amount of gas to aggregate ultrafine particles made of only the metal from the gas phase. The processes of producing ultrafine metal particles also disadvantageous in that it is difficult to keep the ultrafine particles in safe storage because once the solvent is evaporated, the particles stick to each other and cannot be reused.